1. Field of the Invention
The present invention relates to an infrared imaging apparatus, and to an infrared monitoring apparatus for a vehicle for monitoring areas in front of or behind the vehicle using the infrared imaging apparatus.
2. Description of the Related Art
Conventionally, infrared imaging apparatuses for detecting infrared radiation emitted from a target and for outputting image signals are known. These infrared imaging apparatuses are put into practice as infrared monitoring apparatuses for vehicles for monitoring the area around the vehicles.
FIG. 6 is a block diagram showing a structure of such a conventional infrared imaging apparatus. FIG. 7 is a diagram showing an overall configuration of an infrared monitoring apparatus for a vehicle which uses the infrared imaging apparatus shown in FIG. 6.
As shown in FIG. 6, an infrared imaging apparatus 40 comprises a lens 1 for focusing received infrared radiation such as that emitted from a target, and an infrared detector 2 for detecting the infrared focused by the lens 1 and converting the detected infrared radiation into an electrical signal. The electrical signal converted by the infrared detector 2 is amplified by an amplifier circuit 3, converted into a digital signal by an A/D converter 4, and then, converted to an image signal 6 by an image processor circuit 5.
As shown in FIG. 7, an infrared monitoring apparatus 41 for a vehicle comprises the infrared imaging apparatus 40 which is provided at the front of a vehicle 42 and a video monitor 43 for displaying an image based on the image signal 6 output from the infrared imaging apparatus 40. In the infrared monitoring apparatus 41 for a vehicle having such structure, the image signal 6 output from the image processor circuit 5 of the infrared imaging apparatus 40 is input to the video monitor 43 which displays an image. This image is, for example, displayed as a black and white image shaded according to the intensity of the detected infrared. A driver 44 within the vehicle 42 can monitor targets in front of the vehicle 42 by viewing the image displayed on the video monitor 43.
The infrared monitoring apparatus 41 can display targets that are far away and that are difficult, particularly at night, to be seen by the naked eye and thus, the monitored area can be broadened compared to that relying only on the naked eye.
Further, there is a demand for recognizing the distance to a target in the infrared monitoring apparatus 41 for a vehicle because recognition of the distance to a target allows the driver to decide whether or not action should be taken to avoid the target.
However, in the conventional infrared imaging apparatus 40, there is a problem in that it is difficult to recognize the distance to targets. For example, when two people 45 and 46 are present as targets in front of the vehicle 42 as shown in FIG. 7, an image, for example, as shown in FIG. 8 is shown on the video monitor 43. In this case, the driver 44 cannot decide which of the two people 45 and 46 is closer to the vehicle.
This is because it is not possible for the infrared imaging apparatus 40 used in the monitoring apparatus to output an image signal including useful distance information based on the infrared radiation emitted from the target to be detected. More particularly, the infrared radiation emitted from the target is predominantly dependent on the temperature of the target and, thus, the variation in the reception intensity attributable to the distance to the targets is small.
Also, in a typical infrared imaging apparatus 40, the sensitivity of the infrared detector 2 is tuned so that targets at a greater distance can also be detected. However, when this is done, the detected intensity of infrared radiation emitted from targets does not vary significantly even if there is a difference in the distance to the target from the infrared detector 2. Therefore, the infrared detector 2 cannot output an image signal including distance information based on the received signal of infrared radiation emitted from the target.
Furthermore, in order to detect targets at a greater distance, the infrared imaging apparatus 40 in some cases is designed so that the infrared detector 2 detects infrared light in a wavelength range of, for example, 3 μm˜5 μm or 8 μm˜13 μm in which transmittance in air is high, from among various infrared radiation emitted from a target. In such cases, because the attenuation of the infrared emitted from the target and transmitted through the air is small, the difference in the detection intensity of the infrared radiation from targets at different distances is small and, thus, the infrared detector 2 cannot output an image signal including information on the distance to the target based on the received signal.
The present invention was conceived to solve at least the above described problems, and one object of the present invention is to provide an infrared imaging apparatus for outputting an image signal in which the distance to the target can be recognized, that is, an image signal including distance information.